This invention relates to means for separating a copying paper from a sensitive body in an electronic copying machine.
Referring to an example of a conventional electronic copying machine as shown in FIG. 1, a drum-type sensitive body 1 of which surface is made of a layer of a non-crystalline selenium or the like is rotated to a direction P as shown by arrow and charged uniformly by a Corona charged device 2. (The selenium layer has positive electricity.) Following to this, an image of an object (to be copied--not illustrated) which is placed on the sensitive drum 1 is projected on the sensitive drum 1 by exposure from a direction Q (light source). The projected image thereupon is developed by a developing means 3 using a magnetic brush or the like. The developed image is, as the sensitive drum 1 is further rotated, copied on a copying paper 4 which is fed at the same speed as synchronized with the rotation speed of the sensitive drum 1. The voltage of the copying paper 4 as well as of the sensitive drum 1 is decreased by a charged device 6 for removing electric charges, and the copied paper is fed to a final receiving place (not illustrated). On the other hand, the sensitive drum 1 is cleaned by a cleaning device 7, thereby any remaining image on the sensitive drum 1 is removed.
During the process of removing electric charges, that is, during the process of separating the copying paper 4 from the sensitive drum 1 by decreasing the voltage of the copying paper 4 and of the sensitive drum 1, a certain voltage removing electric charge is applied to the copying paper 4 by the charged device 6 for removing electric charges, so that there occurs a potential difference between the copying paper 4 and the sensitive drum 1. Then, electrostatic adsorption is caused therebetween, thereby separation of the former 1 from the latter 4 being difficult.
It is considered that this bad separation is caused by the following reason. In a typical example, it is now supposed that a superficial electric potential of +700 V is always applied to the sensitive drum 1 by Corona discharging of the Corona charged device 2. Then, the decrease of electric potential due to attenuation is not considered. In this case, the superficial electric potential on the surface of the sensitive drum 1 is varied in accordance with respective white portion and black portion of the object to be copied by projecting an image during the exposure process. Due to the exposure, the white portion remains +100 V and the black portion remains +700 V. Under this condition, the staticelectric invisible image on the sensitive drum 1 becomes a visible image by a developing process. Then, the electric potential on the surface of the sensitive drum 1 is attenuated by passing through the developing device 3, i.e. the white portion and the black portion are attenuated to 75 V and 420 V respectively. A visible image on the sensitive drum 1 is further rotated and passed over the charged device 5 in order to copy the visible image on the copying paper 4 which is fed at the same speed as synchronized with a rotation speed of the sensitive drum 1.
In this case, the superficial electric potential of the same polarity, that is, positive electric charges are applied to both the sensitive drum 1 and the copying paper 4 by means of the charged device 5 for copying. As a result, the visible image on the sensitive drum 1 is transferred to the copying paper 4.
A superficial electric potential on the sensitive drum 1 produces an important effect on separating the copying paper 4 from the sensitive drum 1. Based upon the aforementioned example, now 7,000 volts for copying is applied to the white portion and the black portion which have been already developed. In this case, the following data will be given.
______________________________________ Potential differ- ence between the Superficial electric Voltage of sensitive drum 1 potential before the charged and the charged copying device 5 device 5 (Voltage) for copying (Voltage) ______________________________________ White portion 75 7000 6925 Black portion 420 7000 6580 ______________________________________
As shown above, the electric potential of the white portion is larger than that of the black portion. Accordingly, by applying 7,000 V from the charged device 5, the superficial electric potential of the copying paper 4 is changed as follows:
Superficial electric potential of the white portion PA0 Superficial electric potential of the black portion
+395 V (320 V added to 75 V) PA1 +610 V (190 V added to 420 V)
As understood obviously, the increase rate of the electric potential of the white portion is larger than that of the black portion.
When the copying paper 4 is fed over the charged device 6, an AC voltage for removing electric charges, for example, 5,000 V is applied to both the copying paper 4 and the sensitive drum 1 by means of the charged device 6 for removing electric charges. Since negative electric charges contribute to removing electric charges, the following data will be given.
______________________________________ Superficial Potential differ- AC Voltage Voltage of the ence (V) between the for removing sensitive drum 1 sensitive drum 1 and electric and copying paper the charged device 5 charges 4 for copying ______________________________________ White portion 5,000 395 5,395 Black portion 5,000 610 5,610 ______________________________________
As obviously shown above, it is to be understood that the electric current (for removing electric charges) flows to the black portion easier than to the white portion. Therefore, there is the tendency that the electric-charges-removing quantity for the copying paper 4 is increased more than the quantity of electric charges. For the white portion, the electric-charges-removing quantity is inclined to be decreased than the quantity of electric charges.
Accordingly, when the same electric-charges-removing voltage as applied to the white portion is applied continuously to the black portion (in the above example an AC 5,000 Voltage for removing electric charges is applied), after a positive superficial electric potential of the sensitive drum 1 has been attenuated, an electric potential of reverse polarity (negative electric potential in this example) occurs and begins increasing. Then, an electrostatic adsorption between the copying paper 4 and the sensitive drum 1 is again started. This is a main cause a bad separation of the copying paper 4 from the sensitive drum 1.